Fluorescent rare earth complex and security medium using the same

ABSTRACT

The embodiments provide a fluorescent rare earth complex having strong emission intensity and excellent durability, and also provide a security medium using the complex. The rare earth complex according to the embodiment comprises a rare earth ion, a diphosphine dioxide ligand and a β-diketone ligand wherein two phosphorus atoms contained in the diphosphine dioxide ligand individually have substituents different from each other.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2019-163001, filed on Sep. 6,2019, the entire contents of which are incorporated herein by reference.

FIELD

Embodiments of the present disclosure relate to a rare earth complexexcellent in emission intensity and durability, also to a compositioncontaining the rare earth complex, and further to a security mediumprovided with the rare earth complex.

BACKGROUND

Rare earth complexes characteristically absorb light in the UVwavelength region and exhibit sharp emission spectra, and it is hopedthat they will be exploited in many fields, such as, light-emittingdevices, sensors, and security media. In order to employ rare earthcomplexes in those applications, they are required to be excellent inemission intensity, solubility in polymers and solvents, and durability.Phosphors having those favorable properties have been under research anddevelopment, and it has been demanded to provide a phosphor havinghigher performance than conventional ones.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows fluorescent spectra of Example 1 and Comparative example 1.

FIG. 2 shows excitation spectra of Example 2 and Comparative example 2.

FIG. 3 is a graph showing results of durability tests in Example 3 andComparative example 3.

FIG. 4 shows quantum yields of Example 1 and Comparative example 4.

DETAILED DESCRIPTION

Embodiments will now be explained with reference to the accompanyingdrawings.

A rare earth complex according to the embodiment comprises:

a rare earth ion,

a diphosphine dioxide ligand, and

a β-diketone ligand wherein two phosphorus atoms contained in saiddiphosphine dioxide ligand individually have substituents different fromeach other.

Further, a composition for forming a security medium according to theembodiment comprises the above rare earth complex and a polymer.

In addition, a security medium according to the embodiment comprises asubstrate and the above rare earth complex provided thereon.

<Rare Earth Complex>

The rare earth complex according to the embodiment comprises a rareearth ion, a diphosphine dioxide ligand, and a β-diketone ligand. Thisrare earth complex is a phosphor that absorbs UV light and then emitsfluorescence.

In the embodiment, the rare earth ion can be so selected as to give offemission at the wavelength appropriate to the aimed application, but ispreferably a lanthanide ion. Specifically, europium and terbium arepreferred, and europium is particularly preferred in view of obtaining aphosphor complex showing such a large spectrum in the red region asrealizes excellent color rendering properties.

In the structure of the diphosphine dioxide ligand contained in the rareearth complex according to the embodiment, there are two P═O bondsthrough which the ligand is coordinated to the rare earth ion. Twophosphorus atoms in the two P═O bonds can individually have varioussubstituents. In the embodiment, the substituents individuallyconnecting to the two phosphorus atoms are different from each other.This means that the diphosphine dioxide ligand has an asymmetrical form.The asymmetrical form distorts the ligand field and consequentlyenhances the quantum yield. The emission intensity of the rare earthcomplex is proportional to the product of the quantum yield and theabsorption coefficient, and accordingly the emission intensity can beenhanced in proportion to the quantum yield. Further, if the diphosphinedioxide ligand has an asymmetrical form, the complex tends to have highsolubility in solvents and the like.

Each phosphorus atom can have two substituents. In the embodiment, asubstituent set connecting to one of the two phosphorus atoms isdifferent from another substituent set connecting to the other. Forexample,

-   (i) one phosphorus atom has an aliphatic hydrocarbon group and an    aromatic hydrocarbon group while the other has two aliphatic    hydrocarbon groups;-   (ii) one phosphorus atom has an aliphatic hydrocarbon group and an    aromatic hydrocarbon group while the other has two aromatic    hydrocarbon groups; or otherwise-   (iii) one phosphorus atom has two aliphatic hydrocarbon groups while    the other has two aromatic hydrocarbon groups.

In the embodiment, the “aliphatic hydrocarbon group” here means not onlya substituent group consisting of only carbon and hydrogen atoms butalso a group containing an oxygen, nitrogen or sulfur atom, such as, asubstituent containing a hydroxy or amino group. Also, the “aromatichydrocarbon group” in the embodiment may be a group in which thearomatic ring contains a nitrogen or sulfur atom as a ring member. Forexample, the aromatic hydrocarbon group may be a pyridyl group or athienyl group. Further, the aromatic hydrocarbon group may have asubstituent containing an oxygen, nitrogen or sulfur atom, such as, asubstituent containing a hydroxy or amino group.

For the purpose of preparing the asymmetrical diphosphine dioxideligand, it is possible to use, for example, a combination of aliphatichydrocarbon groups having different numbers of carbon atoms. However, inview of enhancing the distortion of the ligand field, it is preferred toform the asymmetry by use of a combination of aliphatic hydrocarbongroups and aromatic hydrocarbon groups. Specifically, the asymmetricaldiphosphine dioxide ligand preferably contains at least one aliphatichydrocarbon group connecting to one of the two phosphorus atoms and atleast one aromatic hydrocarbon group connecting to the other.Particularly preferably, two aliphatic hydrocarbon groups connect to onephosphorus atom while two aromatic hydrocarbon groups connect to theother (this corresponds to the (iii) described above).

The two substituents connecting to each phosphorus atom may linktogether to form a ring.

The β-diketone ligand contained in the rare earth complex according tothe embodiment can be freely selected from known ones. The β-diketoneligand not only serves as a counter ion to the cationic rare earth ion,but also contributes to formation of the ligand field in the complex. Inthe embodiment, a particular β-diketone ligand is adopted so as toobtain a rare earth complex having high durability.

The β-diketone ligand according to the embodiment preferably has ahalogenated hydrocarbon group because it enhances the emissionintensity. The reason thereof is that the halogenated hydrocarbon groupis so electron-withdrawing that the energy difference (ΔE) between theexcited triplet state of the β-diketone ligand and the excited statelevels ⁵D₀ and ⁵D₁ of the trivalent rare earth ion is enough reduced toenhance the efficiency of energy transfer from the ligand to thetrivalent rare earth ion. The linking group thereof normally attaches tothe 1- and/or 3-position of the β-diketone skeleton. If having anasymmetrical structure, the β-diketone ligand also tends to enhance theemission intensity.

Although containing the β-diketone ligand serving as a counter ion tothe rare earth ion, the rare earth complex of the embodiment may containanother anion as the counter ion. Examples of the anion include halogenion, hydroxy ion and carboxylate ion.

The rare earth complex is preferably represented by the followingformula (1):

wherein

each Ar is independently an aryl group,

each R is independently a straight or branched alkyl group,

each R^(F) is independently a halogenated hydrocarbon group,

n is an integer of 3 to 5,

p is an integer of 1 or 2,

q is an integer of 1 to 3,

M is a rare earth ion, and

X is absent when q is 3 but is an anion having a valence of 3-q when qis 1 or 2. This complex is a rare earth ion, such as europium (III) ionor terbium (III) ion, coordinated with a diphosphine dioxide ligand anda β-diketone ligand. Specifically, the complex represented by thefollowing formula (1a), in which the rare earth ion is europium (III),is particularly preferred because it shows high emission intensity.

(wherein Ar, R, R^(F), n, p and q are individually the same as thosedescribed above.)

In the formula (1), two aryl groups connect to one of the twophosphineoxides (P═O) while two alkyl groups connect to the other. Sincethe complex has such an asymmetrical structure, the emission intensitycan be improved.

The two aryl (Ar) groups in the formula (1) may be the same as ordifferent from each other. There are no particular restrictions on thearyl group, but the number of carbon atoms contained therein ispreferably 6 to 20, more preferably 6 to 12, in view of the solubilityand durability of the rare earth complex. The aryl group may havesubstituents such as hydroxy and alkoxy groups as long as they do notimpair the effect of the embodiment. Specifically, the aryl group ispreferably phenyl, 2-methylphenyl, 1-ethoxyphenyl or naphthyl, morepreferably phenyl. If the aryl group is a phenyl group having asubstituent at the 2-position, the solubility tends to be enhanced. Thetwo Ar groups may link together to form a cyclic structure containingthe phosphorus atom. Specifically, for example, it may be a structure inwhich phenylene groups connect to a phosphorus atom via two hydrocarbonchains.

In the formula (1), the two alkyl (R) groups may be the same as ordifferent from each other. There are also no particular restrictions onthe alkyl group, but the number of carbon atoms contained therein ispreferably 3 to 10, more preferably 4 to 8, in view of the solubilityand durability of the rare earth complex. The alkyl group may havesubstituents such as hydroxy and alkoxy groups as long as they do notimpair the effect of the embodiment. Specifically, examples of the alkylgroup include n-butyl, 1-methylpropyl, t-butyl, n-hexyl, 2-ethylbutyl,and n-octyl. The embodiment includes the complex in which the two Rgroups link together to form a cyclic structure containing thephosphorus atom. Specifically, for example, it may be a heterocyclicstructure in which alkylene chains, particularly, alkylene chains of 3to 10 carbon atoms connect to one phosphorus atom.

The two phosphorus atoms in the formula (1) link together via analkylene chain having n carbon atoms. The integer n is 3 to 5,preferably 4. For realizing high durability, the integer n must bewithin that range. If it is 2 or less, the positional relation betweenthe two phosphineoxides is so restricted as to weaken the coordinatebond of the diphosphine dioxide ligand and consequently the emissionintensity is lowered. On the other hand, if the integer n is 6 or more,the diphosphine dioxide ligand may singly connect with two rare earthions to form a dinuclear complex, which lowers the solubility.

The β-diketone ligand in the formula (1) has two halogenated hydrocarbongroups R^(F)s at the 1- and 3-positions. Since the β-diketone ligand hashalogenated hydrocarbon groups, when the complex absorbs light, energycan transfer from the excited ligand to the rare earth ion with suchhigh efficiency as to realize high emission intensity. There are noparticular restrictions on how many carbon atoms the halogenatedhydrocarbon group has, but it preferably has 8 or less, more preferably1 to 3 carbon atoms in view of realizing high solubility. Thehalogenated hydrocarbon group is preferably a perfluoroalkyl group, andis more preferably CF₃, C₂F₅ or C₃F₇. The two R^(F)s may be the same asor different from each other, but are preferably different so that theβ-diketone ligand may be asymmetrical to increase the ligand fielddistortion enough to enhance the emission intensity.

It is generally known that rare earth complexes are improved in lightresistance and quantum yield if the two substituents of the β-diketoneligand are perfluoroalkyl groups. On the other hand, however, it is alsoknown that complexes having that structure have poor solubility inpolymers and/or solvents and hence that compositions containing thosecomplexes (i.e., solutions of the complexes dissolved in polymers and/orsolvents) often have poor transparency. The complexes impaired intransparency are unfavorable for application to security media. In theembodiment, the diphosphine dioxide ligand has such a particularstructure as to realize both that the complex can keep high solubilityand that the composition thereof can have high transparency.

In the rare earth complex according to the embodiment, the β-diketoneligand as well as the diphosphine dioxide ligand is preferablyasymmetrical. The inventors think the reason of that as follows.

When coordinating to the rare earth ion, the β-diketone releases aproton at the active methylene moiety to be in an anionic state andthereby electrically neutralizes the rare earth ion. Accordingly, if theproton returns back to the β-diketone anion and thereby brings it againinto the neutral charge state, the β-diketone ligand dissociates fromthe rare earth ion and consequently the rare earth complex is quenched.This means that the dissociation tendency of the β-diketone ligand fromthe rare earth ion increases according as the neutralizing stabilizationenergy increases on the basis of comparison between the anionic stateand the neutral charge state.

In view of the above, it was research how much the anionizingstabilization energy of β-diketone is affected by substituents.Specifically, under the condition that the substituents (namely, R^(a)and R^(b) in the above formula) were changed, the stabilization energyE_(s) obtained by attaching a proton onto the anion was calculatedaccording to the density functional theory. The results are shown in thefollowing table.

R^(a) R^(b) E_(S) Me Me 351 t-Bu t-Bu 349 CF₃ CF₃ 314 C₃F₇ t-Bu 328 C₃F₇C₃F₇ 310 C₃F₇ CF₃ 312 C₃F₇ Naph 327 CF₃ Naph 329

In the table, Me, t-Bu and Naph are methyl, t-butyl and naphthyl,respectively.

The above results indicate that the set of (Me, Me) or (t-Bu, t-Bu)provides the largest stabilization energy E_(s) and hence that theβ-diketone ligand having them tends to dissociate easily.

When the set is (C₃F₇, t-Bu), (C₃F₇, Naph) or (CF₃, Naph), thestabilization energy decreases considerably. However, since those setsinclude non-perfluoroalky substituents, the complex has insufficientlight resistance. When both substituents are perfluoroalkyl groups,namely, when the set is (CF₃, CF₃), (C₃F₇, C₃F₇) or (C₃F₇, CF₃), thestabilization energy is as small as 310 to 314 kcal/mol. This indicatesthat the β-diketone ligand having them hardly dissociates.

However, if the set is (CF₃, CF₃), the complex tends to have poorsolubility in solvents. In contrast, the set of (C₃F₇, C₃F₇) can improvethe solubility because the substituents provide such an effect of sterichinderance as to reduce interactions among the solute molecules. Thesolubility can be also improved by the set of (C₃F₇, CF₃) because of itsasymmetrical molecular structure.

In general, a phosphine oxide ligand and a rare earth ion function as aLewis base and a Lewis acid, respectively, to form a coordinate bond. Ascompared with when two phosphine oxide ligands coordinate, dissociationtends not to occur when one diphosphine dioxide ligand does because ofthe chelate effect. However, if the diphosphine dioxide ligand has asymmetrical structure, the quantum yield is often so limited that therare earth complex may exhibit insufficient emission intensity. Sincehaving an “asymmetrical diphosphine dioxide”, the rare earth complex ofthe embodiment realizes higher quantum yield.

As described above, the most favorable rare earth complex according tothe embodiment comprises a β-diketone ligand in which the substituentsare (C₃F₇, C₃F₇) or (C₃F₇, CF₃) and an asymmetrical diphosphine dioxideligand in which one phosphorus atom has two aliphatic hydrocarbon groupsand the other phosphorus atom has two aromatic hydrocarbon groups.

The rare earth complex of the embodiment has p diphosphine dioxideligands and q β-diketone ligands provided that p is an integer of 1 or 2and q is an integer of 1 to 3. The most typical complex has onediphosphine dioxide ligand (p=1) and three diketone ligands (q=3) andhence has an 8-coordinate structure. In this case, the charge of therare earth ion is cancelled out with three β-diketone ligands andaccordingly the counter ion X is absent.

The complex may have two or less β-diketone ligands. In that case, twodiphosphine dioxide ligands can generally coordinate but the counter ionX is necessary so as to cancel out the charge of the whole complex. Thatcounter ion may be either one divalent anion or a set of two monovalentanions.

The rare earth complex of the embodiment may further contain a ligandother than the diphosphine dioxide ligand or the β-diketone ligand aslong as the effect of the embodiment is not impaired.

Examples of the rare earth complex according to the embodiment includethe following (1-1) to (1-42). Among them, (1-1) to (1-12) areparticularly preferred.

Xa: counter ion or other ligands

In addition to the above, two or more of the above rare earth complexescan be used in combination as a mixture, which also has as excellentproperties as each complex of the embodiment. Specifically, for example,preferred is a mixture of two or more rare earth complexes comprising

rare earth ions,

diphosphine dioxide ligands, and

β-diketone ligands wherein

said mixture contains at least two rare earth complexes selected fromthe group consisting of:

-   (a) a first rare earth complex comprising

said rare earth ion,

one of said diphosphine dioxide ligands, and

three of said β-diketone ligands;

-   (b) a second rare earth complex comprising

said rare earth ion,

two of said diphosphine dioxide ligands, and

two of said β-diketone ligands; and

-   (c) a third rare earth complex comprising

said rare earth ion,

one of said diphosphine dioxide ligands, and

two of said β-diketone ligands provided that

two phosphorus atoms contained in each said diphosphine dioxide ligandindividually have substituent sets different from each other.

In the synthesis process of the rare earth complex, rare earth complexesdifferent in numbers of the diphosphine dioxide ligands and theβ-diketone ligands are often produced as a mixture. However, it isunnecessary to separate and isolate the complexes from the mixture, andthe mixture can be directly used and hence the production process can bevery simplified. In view of that, the rare earth complex mixture ispreferably adopted.

Further, the rare earth complex of the embodiment may be a polynuclearcomplex which comprises two or more rare earth ions and in which tworare earth ions are bridged with one diphosphine dioxide ligand.Specifically, one diphosphine dioxide ligand in a complex (mononuclearcomplex) represented by the formula (1) connects to one rare earth ion.In contrast, in a polynuclear complex, one diphosphine dioxide ligandconnects to two different rare earth ions, so as to form a binuclearcomplex containing two rare earth ions or a metal organic frameworkhaving a straight chain or branched chain structure in which three ormore rare earth ions are connected via one diphosphine dioxide ligand.The rare earth complex of the embodiment may have those structures.Since having two phosphorus atoms linked together with a single-bondcarbon chain, the diphosphine dioxide ligand in the embodiment tends toform a bridge between two metal atoms.

<Composition for Forming a Light Wavelength-Converting Medium>

The rare earth complex of the embodiment characteristically has largesaturation solubility in media, such as, solvents and polymers.Accordingly, the complex can be dissolved in, for example, a polymer toprepare a composition with which an appropriate substrate is coated toproduce a light wavelength-converting medium, such as an ornament or asecurity medium, which is colorless and transparent under room light butstrongly fluorescent under UV or near UV light. Specifically, forexample, a composition comprising the rare earth complex of theembodiment dissolved in a polymer can be used as an ink for printingbar-codes on security cards or the like, to produce security media thatare completely colorless under visible light but strongly fluorescentunder UV light. Those media can ensure higher security because it isdifficult under visible light even to know there are bar-codes thereon.

The composition for producing the light wavelength-converting media canbe prepared by dissolving the rare earth complex of the embodiment in anappropriate solvent or polymer. Since having high durability orstability, the complex of the embodiment is stable even when dissolvedin a solvent or polymer and accordingly hardly decomposes even whendissolved in a strongly acidic or basic solvent. The solvent can besuitably selected from known organic solvents and the like. Examplesthereof include: esters, such as, ethyl acetate; fluorinated aliphatichydrocarbons, such as, perfluoropentane; aromatic hydrocarbons, such as,toluene and xylene; and alcohols, such as, ethanol, methanol,isopropanol and butanol.

When the rare earth complex of the embodiment is dissolved in a polymer,it is preferred to use a transparent resin such as acrylic resin,polyester resin or vinyl chloride-vinyl acetate copolymer resin. Asdescribed above, since being stable even under a strongly acidiccondition, the rare earth complex of the embodiment can keep highdurability even when dispersed or dissolved in an acidic resin. In viewof that, the embodiment is advantageous.

The acidic resin, for example, has a carboxyl group. Example thereofinclude: alkyd resins, such as, linseed oil-modified maleic alkyd resinand acryl-modified alkyd resin; and polyacrylic resin. The acidic resinmay have sulfonic acid, such as, polystyrene sulfonic acid. Further,many acidic cation-exchange resins can be adopted as the acidic resin.

According to necessity, the composition may contain additives, such as,surfactant, antifoaming agent, antiseptic agent and pH adjuster. Whenused for security media, the composition according to the embodiment isgenerally preferred to have low visibility and hence to be colorless andtransparent under visible light. However, if necessary, dyes or pigmentscan be incorporated therein. Further, the rare earth complex can beenclosed in microcapsules so as to keep stably.

<Light Wavelength-Converting Medium>

The light wavelength-converting medium, such as a security medium,according to the embodiment comprises a substrate and the above rareearth complex provided thereon. The medium having that structure can beproduced by coating a substrate with the above composition. Thesubstrate may be made of any material, such as paper, plastics, cloth,nonwoven or ceramics. There are no particular restrictions on thecoating method, but in general a specific image is printed ortransferred onto the substrate to form a distinctive mark.

In another way, the rare earth complex is dissolved in a polymer andthen the polymer is cured to be a substrate. By use of that substrate,it becomes possible to produce a security medium in which the substrateitself emits light under UV light.

EXAMPLES

The following examples will explain the embodiments, but they by nomeans restrict the embodiments.

Example 1

The Eu(III) complex represented by the formula (1-1) was dissolved inethyl acetate (2×10⁻⁴ mol/L), and the emission spectrum thereof wasmeasured. FIG. 1 shows the obtained spectrum. The maximum absolutequantum yield of the complex (1-1) was found to be 0.47 in the solution.

Comparative Example 1

The emission spectrum of the Eu(III) complex represented by the formula(R-1) was measured in the same manner as in Example 1. The maximumabsolute quantum yield of the complex (R-1) in the solution was found tobe 0.40, which indicates that the emission intensity of the complex(R-1) is weaker than that of the complex (1-1).

Example 2

The excitation spectrum of the solution prepared in Example 1 wasmeasured. FIG. 2 shows the obtained spectrum.

Comparative Example 2

The excitation spectrum of the solution prepared in Comparative example1 was measured. FIG. 2 shows the obtained spectrum, which indicates thatthe excitation intensity of the complex (R-1) is weaker than that of thecomplex (1-1).

Example 3

The solution prepared in Example 1 was applied on a sheet of paper,dried and then subjected to a light-resistance test according toJISB7754 “light and weathering resistance test with a xenon arc lamp”.As shown in FIG. 3, the luminance retention ratio of the complex (1-1)after 24 hours was found to be as high as 72%.

Comparative Example 3

The solution prepared in Comparative example 1 was applied on a sheet ofpaper, dried and then subjected to the light-resistance test in the samemanner as in Example 3. As shown in FIG. 3, the luminance retentionratio of the complex (R-1) after 24 hours was 17%, from which it wasfound that the complex (R-1) is much inferior to the complex (1-1).

Example 4

The procedure of Example 1 was repeated except for using the complexrepresented by the formula (1-8) as the phosphor to evaluate theabsolute quantum yield and light resistance in the solution. As aresult, it was found that the absolute quantum yield of the complex(1-8) is about 1.2 times as high as that of the complex (1-1) and thatthe light resistance of the complex (1-8) is the same as or better thanthat of the complex (1-1).

Comparative Example 4

The complexes (1-1) and (R-2) were evaluated as phosphors on the quantumyield and light resistance. First, with respect to each of the complexes(1-1) and (R-2), the absolute quantum yield in the solid crystal statewas measured while changing the wavelength. FIG. 4 shows the results,which indicate that the absolute quantum yield of the complex (R-2) isabout 0.7 times as large as that of the complex (1-1) in the solidcrystal state. In addition, the fluorescence emitted from the complex(R-2) was almost quenched after 24 hours.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel methods and systems describedherein may be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the methods andsystems described herein may be made without departing from the spiritof the inventions. The accompanying claims and their equivalents areintended to cover such forms or modifications as would fail within thescope and spirit of the invention.

1. A rare earth complex comprising: a rare earth ion, a diphosphine dioxide ligand, and a β-diketone ligand wherein two phosphorus atoms contained in said diphosphine dioxide ligand individually have substituents different from each other.
 2. The rare earth complex according to claim 1, wherein said rare earth ion is a europium (III) ion or a terbium (III) ion.
 3. The rare earth complex according to claim 1, wherein at least one aliphatic hydrocarbon group connects to one of said two phosphorus atoms while at least one aromatic hydrocarbon group connects to the other.
 4. The rare earth complex according to claim 1, wherein said β-diketone ligand has a halogenated hydrocarbon group.
 5. The rare earth complex according to claim 1, which is represented by the following formula (1):

wherein each Ar is independently an aryl group, each R is independently a straight or branched alkyl group, each R^(F) is independently a halogenated hydrocarbon group, n is an integer of 3 to 5, p is an integer of 1 or 2, q is an integer of 1 to 3, M is a rare earth ion, and X is absent when q is 3 but is an anion having a valence of 3-q when q is 1 or
 2. 6. The rare earth complex according to claim 5, wherein said integer n satisfies the condition of n=4.
 7. The rare earth complex according to claim 5, wherein said two R^(F) groups have different chain lengths.
 8. The rare earth complex according to claim 5, wherein said integers p and q satisfy the conditions of p=1 and q=3, respectively.
 9. The rare earth complex according to claim 5, wherein said two Ar groups are both substituted or unsubstituted phenyl groups.
 10. The rare earth complex according to claim 5, wherein said R group is an alkyl group having 3 to 10 carbon atoms.
 11. The rare earth complex according to claim 1, which is a polynuclear complex comprising two or more rare earth ions and having a structure in which two rare earth ions are bridged with said diphosphine dioxide ligand alone.
 12. A rare earth complex mixture which contains two or more rare earth complexes comprising rare earth ions, diphosphine dioxide ligands, and β-diketone ligands wherein said mixture contains at least two rare earth complexes selected from the group consisting of: (a) a first rare earth complex comprising said rare earth ion, one of said diphosphine dioxide ligands, and three of said β-diketone ligands; (b) a second rare earth complex comprising said rare earth ion, two of said diphosphine dioxide ligands, and two of said β-diketone ligands; and (c) a third rare earth complex comprising said rare earth ion, one of said diphosphine dioxide ligands, and two of said β-diketone ligands provided that two phosphorus atoms contained in each said diphosphine dioxide ligand individually have substituent sets different from each other
 13. The rare earth complex mixture according to claim 12, wherein said rare earth ions are europium (III) ions or terbium (III) ions.
 14. A light wavelength-converting medium, comprising the rare earth complex according to claim 1 and a polymer.
 15. A light wavelength-converting medium, comprising the rare earth complex mixture according to claim 12 and a polymer.
 16. A light wavelength-converting medium, comprising a substrate and the rare earth complex according to claim 1 provided thereon.
 17. A light wavelength-converting medium, comprising a substrate and the rare earth complex mixture according to claim 12 provided thereon.
 18. The light wavelength-converting medium according to claim 14, wherein said polymer comprises acrylic resin, polyester resin or a mixture thereof.
 19. The light wavelength-converting medium according to claim 14, wherein said polymer is an acidic resin and said rare earth complex is dissolved or dispersed in said polymer.
 20. The light wavelength-converting medium according to claim 19, wherein said acidic resin is selected from the group consisting of alkyd resin, polyacrylic resin, polystyrene sulfonic acid and acidic cation-exchange resin. 